1. Field of the Invention
This invention relates to the polishing of a silicon substrate and, more particularly, to the removal of copper from a silicon substrate following chemical-mechanical polishing.
1. Description of the Prior Art
One stage in the manufacture of semiconductor devices commonly consists of polishing silicon wafers. A commonly used polishing process is chemical-mechanical polishing. The chemical-mechanical polishing process has already been described in great detail in U.S. Pat. No. 3,436,259 issued Apr. 1, 1969 to Regh et al. entitled "Method for Plating and Polishing a Silicon Planar Surface" (whose teachings are incorporated herein by reference thereto), having the same assignee as the present invention. This patent is hereby incorporated by reference for the sole purpose of providing a background to those of skill in this art. In the chemical-mechanical polishing process, a silicon substrate is wetted with an excess quantity of a displacement plating solution, while it is continually wiped, using substantial pressure, with a firm surface. The displacement plating solution contains a cupric cation which is plated onto the silicon substrate as copper metal. The simultaneous and continuous wiping of the silicon substrate removes the copper from the silicon surface and produces an extremely flat and well-polished surface on the silicon.
Unfortunately, small amounts of copper remain on the polished silicon substrates. It is necessary to remove this copper because copper atoms in silicon effect the silicon's conductivity and can interfere with the operation of the devices manufactured from the silicon substrate.
One method of removing such copper contaminant had been to rinse the silicon surface with large amounts of high purity nitric acid. While this process is successful in achieving an acceptably low level of copper on the surface, it has a number of serious drawbacks. The use of large quantities of high purity nitric acid is expensive. Because of its corrosive nature using high purity nitric acid can be dangerous and creates a serious disposal problem. Large scale processing is made difficult because the nitric acid tends to corrode almost all of the commercially available automated cleaning equipment. The copper contaminant level on the silicon surface is strongly dependent on the purity of the nitric acid used; thus, only very high purity nitric acid is acceptable for cleaning. Another problem is that the nitric acid oxidizes the surface of silicon and produces a thin oxide film there; this oxide film hinders the removal of copper from the silicon.